1. Field of the Invention
This invention concerns a detection system using scintillating materials which emit photons upon absorption of nuclear radiation. More particularly, such materials are employed as sensitive nuclear radiation detectors wherein an organic scintillator embedded in a polymer is fabricated as an optical fiber. The light from the scintillating materials is coupled to an ultraviolet transmitting optical fiber for long distance light transmission to a detector unit.
2. Description of the Prior Art
The purpose of any radiation detector is to convert the ionization produced by radiation into a detectable signal. In the case of gamma ray interactions there are three processes of significance, photoelectric interaction, Compton scattering, and pair production. All gamma ray detectors utilize at least one of these processes to obtain a measurable output signal. Common nuclear detection instruments include gas-filled counters, scintillation counters, and semiconductor detectors.
Gas-filled detectors are widely used for gamma-ray detection. Specific examples are ion chambers, proportional counters and Geiger-Mueller counters. Gamma-rays cause ionization of the gas used in these counters. This ionization, in combination with a high voltage applied to the electrodes located inside the gas chambers, leads to a measureable electrical current flow.
In a scintillation counter the detecting medium is a solid or liquid rather than a gas and the detection efficiency is correspondingly higher because of the increased density of the detecting medium. The most commonly used scintillation material is sodium iodide activated with thallium. In all scintillation systems a volume of scintillator material is viewed by a photomultiplier tube which detects the optical photons emitted from the scintillator as a result of the interaction with gamma-rays.
The principle of operation of a semiconductor detector is similar to that of a gas-filled detector except that the detecting medium is a solid. An applied electric field sweeps out free electrons and holes, producing a depletion layer containing practically no free-charged carriers.
Geiger-Muller tubes, sodium-iodide (NaI) scintillators and germanium drifted or doped lithium (Ge/Li) detectors can be designed to have about the same sensitivity as the device developed according to the present invention. However, for a number of applications such conventional detectors exhibit several disadvantages. All three types of detectors are point sensors which measure radiation at required low levels only within a limited area or point around the immediate vicinity of the detector.
All three detectors also have very delicate electronics directly connected to the radiation sensor. This poses potential problems with regard to their deployment. The integral combination of sensor and process electronics results in a relatively large device which cannot be easily placed. Such devices are also susceptible to electromagnetic interference, dirt, vibration, etc., which further limits their placement.
Lithium drifted germanium detectors are somewhat inconvenient in that they must be continuously stored and operated at cryogenic temperatures in order to preserve the lithium profile or distribution within the germanium. Normally, these detectors are operated in a vacuum chamber with liquid nitrogen cooling. Although a Peltier-type cooler could probably be developed for this detector, its electric power consumption and complexity rule out this detector for the intended applications noted below.